What is a “green-roof?” Some say the term conjures up images of green vegetation on a building’s roof; others refer to the concept of making a building’s roof green from an environmental standpoint with green energy solar panels. It turns out that green vegetation on your roof can actually help boost your roof-mounted photovoltaic panels.

Green building roofs

Keeping a garden on the roof is an eco-friendly solution that helps increase the energy performance of a building as it reduces unwanted heat gains in the summer and heat losses in winter – (see green buildings: Rational solutions). The soil and insulation and waterproofing materials that form the infrastructure for planting a roof garden act as an effective insulation for the building at the place where most energy losses occur – the roof. In summer, radiation heat is reduced as it does not reach the roof, thus does not enter inside the building, whereas, in winter, it acts as an effective insulation that keeps the required internal heat inside. The end result may result in energy savings for the building of up to 30 percent.

In addition, apart from the immediate cooling effect of a roof-garden building, green roofs in a community may lead to a micro-climate cooling effect that may be beneficial to the broader related area and community. Consequently, green roofing may help combat “heat island” effects in urban areas – a term used to describe the accumulation of heat buildup in urban areas that eventually remain hotter than rural surroundings.

Research documented in Green Roof Valuation shows that vegetated rooftops can facilitate solutions of complex environmental problems in urban areas. The report suggests that green (vegetated) rooftops, when compared to conventional rooftops, may yield a Net Present Value (over a period of energy savings for up to 40 years) that is 20–30 percent lower; thus, a viable investment over conventional rooftops.

Specifically, the initial additional investment required to make a roof plantable with green vegetation can in fact be recovered, through energy savings, by the time an alternative conventional roof would need replacing. In addition, findings suggests that green roofs may help improve the quality of the urban air, yielding additional benefits in monetary terms and reducing harmful atmospheric emissions. An illustrative example is that a 2000 square meter green (vegetated) roof is estimated to yield approximately $900 to $3300. Consequently, maintaining well vegetated roof-gardens may lead to significant financial, environmental and aesthetic benefits primarily from saving conventional energy but also through urban air quality improvement, and micro-climate cooling effects.

How green roofs can help boost photovoltaic panels’ performance

In addition to these benefits of maintaining a green roof garden, a green roof can boost the performance of roof-mounted photovoltaic panels. One of the biggest and most significant performance parameters of photovoltaic panels is temperature, thus solar panel manufacturers specify their temperature coefficient. The performance boost by combining a solar roof-garden with solar photovoltaic panels has been the object of various research teams; findings suggest a boost effect around 15-16 percent output more than conventional roof mounted panels (i.e. without green vegetation). The increase in photovoltaic performance is mainly due to the cooling effect of the green garden, which is magnified in hot climates and during the hot summer season. An illustration of this result can also be found by comparing the output performance of a photovoltaic panel when it is installed in urban areas (e.g. on a conventional rooftop) with the corresponding performance when installed in rural areas.

Consequently, when having to make a decision about either of these solutions, it is important to note how the combination of both roof solar panels and a green (vegetated) roof, can offer extended benefits that are financial, environmental and architectural. Combining the energy savings from a roof garden, together with the boost effect on the performance of photovoltaic panels, can help reduce the cost of solar panels (see – how much solar panels cost) , thus making photovoltaic panels financially more viable especially for residential applications (solar panels for home).